It is desirable to eliminate noise from AM radio signals to avoid distracting sounds in the audio broadcast. Automotive radios are particularly susceptible to noises because of their proximity to the engine ignition system, which generously emanates radio signals. The spark of the ignition system often creates a very short duration impulse or spike known as a tic, which is very disconcerting to the listener. It is already known to eliminate the tic by detecting its occurrence and blanking the noise by preventing radio signal passage to the speakers for the duration of the tic, which is typically less than 250 microseconds. Because this particular type of noise has such a short duration, the interruption of the signal is not noticeable if that period is filled with an approximation of the correct signal. There are two critical aspects of such noise blanking: correctly and efficiently detecting the impulse noise, and removing the noise in the optimal manner.
An example of a prior circuit which blanks short duration pulses is shown in U.S. Pat. No. 4,637,066 to Kennedy et al which is conceptually like FIG. 1 herein. An input signal received at an antenna 110 is converted into an IF signal in a front end circuit 112 and is amplified in the IF amplifier 114. The signal is AM detected at the detector 116 to generate the AM mono audio signal. The low pass filter 118 averages the detected signal and presents the average value to the comparator 122. The non-averaged signal is reduced by an attenuator 120 and compared to the average. The comparator generates an output or flag when the attenuated noise spike rises above the average. If the noise is not of short duration, the average signal will soon increase so the difference will decrease and the flag will disappear. When the flag is enabled, the audio signal path is switched off with a switch 128 and a capacitor 124 holds the output constant, thereby suppressing the tic. There is a delay stage 126 incorporated into the signal path. This is needed to synchronize the tic detection circuitry and the signal path. Since it takes a finite time to detect the presence of a tic, the signal path must be delayed to assure the signal is being held at the correct instant in time. This circuit works well for the simplified mono system. There is no processing of the audio signal so the hold time does not have to be varied, and there are no additional signals like those available in AM stereo circuits which can be used as flags. The complexity of the AM stereo system provides more flags for tic detection and requires a more sophisticated hold method.
Once a tic is detected, there must be a way to remove it from the audio signal. It is already known to use a sample and hold circuit to interrupt the audio signal and hold it constant during the period of the tic. For example, there is a sample and hold (S/H) circuit at the deemphasis stage in the FM stereo which has been disclosed in the U.S. Pat. No. 4,975,953 to Kennedy et al entitled "Combined Deemphasis Circuit and Noise Blanker". This placement of the S/H circuit has been carried over into the AM stereo system disclosed herein although there are different reasons for such placement. In the FM case, the input signal has 38 kHz information riding on it, and this will affect the held data. The deemphasis attenuates this information, so it is the ideal place to hold the signal. In the AM system, there is no unrelated high frequency information. The hold is made at the deemphasis to realize the required delay time in the signal path. In both systems, the hold cannot be placed after the deemphasis, because the signal will be narrow band and the impulse will e stretched.